In order to understand the advantages which LOM system offers one has to consider how small batches of parts are usually produced. In conventional manufacturing the part's design is first created using computer aided design (CAD) or other drafting techniques. Later, manufacturing operations are defined and the prototype is painstakingly produced by conventional cutting or forming processes, often requiring skilled labor, considerable time and expense. Multiple tools and machines are used in such production as a rule. After the prototype testing, design changes are likely to occur, and laborious production process has to be repeated until the design is optimized.
Therefore, the ability to manufacture prototypes or small batches of parts directly as a computer output utilizing a single production device is highly desirable. If the modification of the design is needed after the part has been examined, a necessary change can be done on the computer screen and another "hard" copy can be created by the LOM system.
In general, the family of LOM systems proposed herein use laser as a tool for forming laminations and bonding them into a stack. In recent years, flexibility and responsiveness of laser based systems motivated a number of organizations and inventors to apply them in the three-dimensional object production. Several techniques based on two intersecting laser beams selectively solidifying ultra violet (UV) curable liquids at the point of their intersection deep within the liquid medium have been described in U.S. Pat. Nos. 4,041,476, 4,078,229, 4,238,840 and 4,288,861. These systems have suffered from a number of problems related to their resolution, exposure control and difficulties related to synchronous control of two intersecting laser beams.
A more successful process and system has been purposed by Charles W. Hull in U.S. Pat. No. 4,575,330. The stereolithography process described in this patent generates three-dimensional objects by curing a UV curable material with a single laser beam focused on the surface of a platform placed in a vat of a UV curable plastic. As the beam cures a cross-section of the part the platform makes an incremental move down thus exposing another layer of liquid plastic. The beam scans the new surface within the pattern of the desired cross-section solidifying the plastic material within that pattern and attaching it to the previous cross-section. The step is repeated until the desired object is produced.
In spite of a number of advantages gained by this method with respect to earlier technologies the method has a disadvantage of being capable of producing parts out of liquid (mainly UV curable) polymers only. These polymers represent a relatively limited group of materials. They are often toxic. The parts produced through the UV curing process are usually only partially cured and therefore are dimensionally and structurally unstable as they are removed from the vat.
In order to prevent their sinking into the liquid, a substantial support structure has to be designed and built for cross-sections located above the platform and unattached to other cross-sections. The process also suffers from internal stress problems created as a result of a shrinkage caused by the UV curing process within the plastic part. These stresses cause warpage of unsupported or suddenly expanding cross-sections and therefore make it difficult to create certain geometries. Thick walled parts are difficult to create for the same reason. The speed of the process is also limited by the low powers of currently available UV lasers.
Other developments have taken place with the use of powder materials in building near net shape three-dimensional parts. U.S. Pat. No. 4,323,756 of C. Brown, E. Brienan and H. Kear describes a technique for building parts in a layered fashion using high power energy beams to melt substrate surface and added stock. In this technique powder is deposited onto a substrate by blowing a stream of it through a nozzle coaxial with a laser beam heating and melting it along with the substrate as soon as the powder reaches the surface of the substrate. In order to direct the new powder to the desired places of the laminated part either the part or the nozzle have to be moved in a controlled fashion. This method has an obvious disadvantage associated with the necessity to overcome inertia of moving mechanical components. Also, the ability to deposit material in a precise fashion to achieve high resolution in the final product is questionable in this technique, since in order to be deposited the material has to go through a nozzle.
Early concepts related to use of sheet materials in the three-dimensional parts buildup have been explored by Japanese scientists (See: Masonory Kunieda and Takeo Nakagawa "Manufacturing of Laminated Deep Drawing Dies by Laser Beam Cutting", Advanced Technology of Plasticity, Vol. 4 (1984)). Although some methods for laser cutting laminae and joining them together have been described, this work has not suggested ways of using the laminating technique for building a computer driven device which would transfer three-dimensional computer images into physical parts in one automated step.
My earlier U.S. Pat. No. 4,752,352 has suggested a number of methods and systems for accomplishing this goal. The current application relates to significant improvements on the methods and apparatus described in the earlier patent. These apparatus create three-dimensional parts out of substantially planar cross-sections utilizing powder based or sheet materials. The methods overcome the material limitations of the stereolithographical technique by making it possible to use a wide range of powder materials (including metals) as well as many plastic, metal and composite sheet materials for the laminated manufacturing of three-dimensional objects. At the same time they allow to achieve much greater speed and finer resolution than the process disclosed in the Brown et al. '756 patent by avoiding the material deposition through a nozzle, thus, allowing the use of scanning techniques in the energy beam manipulation.
Another type of an automated modelling system based on liquid polymers is being developed by an Israeli company, Cubital (See: Itzhak Pomerantz, "Automated Modeling Machines", NCGA 1989 conference proceedings, Apr. 17-20, 1989). Their system manufactures models out of liquid polymers by a multistep process. The steps of the technique are: deposit a thin layer of a UV curable polymer; illuminate the polymer through a xerographically produced mask having geometry of a single cross-section; suction off the liquid material surrounding the cured cross section; fill the areas surrounding the cross-section with a water soluble UV polymer, water, or wax serving as support; cure the rest of the layer or freeze the water; grind the surface to establish a uniform layer; repeat the earlier steps until the part is complete; thaw the ice, or melt the wax surrounding the part or dissolve the water soluble polymer. The process is very complex but it resolves some geometry problems present in stereolithography. Since the process is based on liquid UV curable polymers it does not resolve material limitations and internal stress and shrinkage problems related to stereolithography.
Still another technique relying on illumination of liquid polymers through a mask is being developed by Efrem Fudim of Light Sculpturing Inc. based in Wisconsin. His U.S. Pat. Nos. 4,752,498 and 4,801,477 describe several techniques which are somewhat similar to the earlier described method being developed by Cubital. These methods usually involve illumination of a UV curable polymer with a UV light through plotter generated masks and a piece of flat material transparent to the UV radiation and remaining in contact with the liquid layer being cured. Although the method is simpler than the Cubital technique and is much more energy efficient than stereolithography it has certain limitations related to unsupported geometries. It also relies on UV curable liquid polymers and, therefore, is limited by the properties (shrinkage, warpage, fragility, strength) and relatively small number of these materials. Still another development similar to my powder based LOM process is taking place at the Desktop Manufacturing Inc. (formerly Nova, Inc.) associated with the University of Texas (Austin). A process called "Selective Laser Sintering" has been under development by this company (See "Desktop Manufacturing" report published by Technical Insights, Inc. in 1988). The technique involves sequential deposition of thin layers of metal or plastic powders and selectively sintering these powders with a scanning laser beam. My work has included conducting an extensive experimental investigation of the powder and sheet LOM processes using a prototype system which I built at John Deere, Inc. Although a significant part of the present patent is dedicated to improvements on the powder technique the current view of the author is that the sheet LOM process may be superior to the powder technique. The powder technique has a serious problem of heat generated internal stresses which distort laminated objects and limit the number of available geometries. Structural properties of parts created out of powder have to date been greatly inferior to the ones created out of sheet.
One more computer automated manufacturing technology related to the presently described development is sometimes called "Ballistic Particle Manufacturing". This is a dot matrix printer like technique for creating 3-D parts (U.S. Pat. No. 4,665,492). In this method particles of molten material are deposited in a controlled fashion to create a three-dimensional part. This method is expected to have some problems related to internal stress and low resolution.
It is presently believed that the sheet based LOM method is the best proprietary technology among the ones described earlier. The majority of the related technologies are based on one or another form of a UV curable technique, Only one other process is related to the fusion of powders with lasers. None of them is based on sheet materials. Advantages of the sheet LOM process as compared to the ones described earlier are as follows:
1) The main competitive advantage of the sheet LOM process is in its ability to make parts out of far more off-the-shelf materials than UV curing techniques. My work has already resulted in the production of parts out of metal, plastic, and paper. The paper based parts have properties similar to plywood. PA1 2) Internal stress is a very serious problem in the stereolithography, Selective Laser Sintering (SLS), and Cubital (Instant Slice Curing) processes. The sheet LOM process produces virtually no internal stress. PA1 3) The sheet technique is much faster than stereolithography or the powder LOM processes. The reasons are as follows: PA1 4) Model production has been chosen as an initial market by many in the field, since UV curable parts created by the 3-D Systems stereolithography process or Cubital's "Instant Slice Curing" technique are rather fragile and can not serve as functional parts yet. By creating several metal and plywood objects my work on the LOM process has demonstrated a clear potential to produce functional parts such as dies, molds, and production parts directly. So far there has not been any proof that laser beam scanning is a good way of producing structurally strong materials (when a laser beam scans a UV or powder cross section it creates the material out of which it will be made). On the other hand the LOM technique just outlines the geometry of a cross section by cutting it around its periphery. This preserves the original properties of the sheet material which has been earlier created by an extrusion, cold or hot rolling, or plastic film production processes. PA1 5) Because of the absence of internal stresses in the sheet LOM technique and the unpredictable shrinkage of parts associated with it there is a potential of manufacturing objects with the XY direction tolerances significantly higher than with internal stress affected techniques. PA1 6) Processes relying on manufacturing out of UV curing polymers have been subject to strict OSHA scrutiny since they involve potentially harmful substances. The majority of sheet materials used in LOM process are considered safe. PA1 7) Although virtually no waste is generated in the UV curing technique the liquid polymers used in it are extremely expensive. Most of the sheet materials are over an order of magnitude cheaper. PA1 8) Due to being a one step process (not requiring post curing operations), and because of high speed of production and lack of internal stresses the sheet LOM process is expected to produce extremely large parts just as efficiently as tiny ones. PA1 9) The only process besides LOM which is capable of automated creation of unsupported (cantilever) geometries of unrestricted complexity is the one being developed by Cubital. However, their process is significantly more complex than the sheet LOM technique.
a) Due to the high viscosity of UV liquids it takes a considerable amount of time to form a layer (even with the newly introduced wiper blade leveling the liquid). Although the curing step is faster in the Cubital process than that of stereolithography a considerable amount of time is required to perform other steps of this multi step technique. Deposition of a sheet in the LOM process can be virtually instantaneous. PA2 b) Because of the fact that during the execution of the sheet process the laser beam outlines the periphery of a cross-section instead of raster painting its complete area as is done in UV or powder processes thin walls are produced just as fast as thick ones. The only factor that matters is the periphery of a cross-section. Therefore, extremely large parts with thick walls (even ones which would be difficult to mold or cast) can be produced by this technique.